1. Whisk together lime juice and olive oil, add jalapeño, mint and scallions and mix until well combined. Season to taste with salt and pepper.
2. Cut the watermelon in half and place the cut side down on a cutting board. Using a sharp knife carefully remove the rind, then cut the watermelon into ½ inch cubes.
3. Place cubed watermelon in a bowl and toss with dressing, season to taste with salt.